One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to generally as chemical vapor deposition (“CVD”). Conventional thermal CVD processes supply reactive gases to the substrate surface, where heat-induced chemical reactions take place to produce a desired film. Plasma-enhanced CVD (“PECVD”) techniques, on the other hand, promote excitation and/or dissociation of the reactant gases by the application of radio-frequency (“RF”) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes as compared with conventional thermal CVD processes. These advantages are further exploited by high-density-plasma (“HDP”) CVD techniques, in which a dense plasma is formed at low vacuum pressures so that the plasma species are even more reactive.
Any of these CVD techniques may be used to deposit conductive or insulative films during the fabrication of integrated circuits. For applications such as the deposition of insulating films as premetal or intermetal dielectric layers in an integrated circuit or for shallow trench isolation, one important physical property of the CVD film is its ability to fill gaps completely between adjacent structures without leaving voids; this property is referred to as the film's gapfill capability. Gaps that may require filling include spaces between adjacent raised structures such as transistor gates, conductive lines, etched trenches, or the like.
As semiconductor device geometries have decreased in size over the years, the ratio of the height of such gaps to their width, the so-called “aspect ratio,” has increased dramatically. Gaps having a combination of high aspect ratio and a small width present a particular challenge for semiconductor manufacturers to fill completely. In short, the challenge usually is to prevent the deposited film from growing in a manner that closes off the gap before it is filled. Failure to fill the gap completely results in the formation of voids in the deposited layer, which may adversely affect device operation such as by trapping undesirable impurities. The semiconductor industry has accordingly been searching aggressively for techniques that may improve gapfill capabilities, particularly with high-aspect-ratio small-width gaps.